Some forum systems (not Discourse) do both — topic creator’s avatar on the left, next to the post title and creation date (left-justified, assuming a LTR locale), then the latest poster’s avatar at the right edge, with the timestamp for their post underneath.
That’s kind of ideal, in my experience. It associates every data element with the ones it’s most relevant to, and clearly separates not-directly-related items (like the title and the last posting time).